How to animate particle size (Newtonian Physics)
If you go to the Physics>size tab under Newtonian, and either hover your mouse over the input box, and press  RMB>insert keyframe, or just press  I while hovering over the input box after setting the particles to the size you need for your scene. Do the same thing to set the scaling down at your later frame. In my example, it is 30 frames using a sphere as a dupli object.